This artical was written in 2005. Currently (2018-19) the Trump Administration has proposed re-writing many of the climate-change regulations of Clean Air Act of 1970 and its 1990 Amendment.

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Many of us learned about acid rain as school children or watched it being broadcasted on the news. Even though there was a lot of talk about acid rain during the 90’s, today we hear even less about it. To many of us, it seems a distant and unimportant problem of the past, and on the surface, it seems to have all but disappeared. Higher standards of pollution control have indeed made great strides in combating the toxic rain, but much of the damage done to the soil will take years to reverse while pollution continues to occur. What exactly is the problem of “acid rain” today?

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It is easy to imagine acid rain as toxic green drops of water, but in reality acid rains occurs in several forms (and is not particularly green). According to the U.S. Environmental Protection Agency, "acid rain is a broad term used to describe several ways that acids fall out of the atmosphere. A more precise term is acid deposition, which has two parts: wet and dry.” In its wet form aciddeposition can appear as acidic fog, snow, or rain. The wet forms are dangerous because they penetrate the soil as water washes over the ground and can harm many plants and animals.

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Dry deposition is acidic gases and particles. Nearly half the acidity in the atmosphere falls back to the earth as dry deposition. This type of acidity is carried by the wind often bombarding statues, buildings, cars, and trees. In addition rain can wash off the gaseous acid that builds up on trees and buildings—a far more powerful combination than acid rain alone. When it rains acid, it pours acid!

In either it’s wet or dry forms, acid rain can travel a great distance in the atmosphere. In fact, prevailing winds carry the aciddeposition hundreds of miles, circulating acid rain between neighboring countries like the United States and Canada.

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The primary cause of this acid in the atmosphere is sulfur dioxide, SO2, and nitrogen oxides, NOx.

In the U.S. approximately 2/3 of all
SO2 comes from the burning of fossil fuels like coal. The graphs shown to the left detail the primary sources of SO2 emissions in both Canada and the U.S.

The graph below shows how the levels of SO2 have changed over the last 20 years in both countries. In 1998, U.S. SO2 emissions were measured at 17.7 million metric tons - more than six times greater than Canada's 2.7 million metric tons. Notice that in recent years the U.S. emissions have decreased dramatically, and are now at about 15 million metric tons/year.

It should be noted that a metric ton, or tonne, is the weight of 1000 kg at sea level or approximately 2200 lbs. An American ton is only 2000 lbs.

The graphs shown to the left detail the sources of
NOx emissions in Canada and in the U.S. In both the US and Canada, the main source of NOx emissions is the combustion of fossil fuels in cars. Notice how the remaining sources of pollution differ between the two countries.

Overall, NOx emissions amounted to 2.1 million metric tons in Canada in 1998. U.S. NOx emissions for 1998 totaled at 23.7 million tonnes-11 times more than Canada's. This is in part due to the huge population differences between the US and Canada. The graph below shows how the levels of NOx have changed over the last 20 years in both countries.

Despite the large amounts of pollution, a lot is being done to prevent emissions. By 2010, the government-supported Acid Rain Program hopes to reduce annual SO2 emissions to half of their 1980 levels. The program also sets a limit on the total amount of SO2 power plants can emit nationwide, capping it off at 8.3 million tons. There are controls on NO2 emissions as well. In addition, the program has built-in economic incentives that allow companies to trade pollution credits. Each source has an allowance for the amount of pollution it may emit, and if it is under budget, it can bank or sell its pollution credits. This approach allows the companies flexibility in reducing their emissions in the most cost-effective way.

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Acids, like those produced by NOx and SO2, have several defining characteristics. They are sour in taste, and will react with bases to form neutral salts and water. Acids also change the color of litmus paper (used by scientists to measure pH) from blue to red. The strongest acids are capable of burning the skin.

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The way scientists measure the strength of acids is called the pH scale. Water, a neutral substance, rates as a 7 on the scale. Substances with a number lower than 7 are acids. Typically, rain with a pH of 0-5 is labeled acid rain.

Although the scale is small, the difference between a pH of 5 and 6 is extreme. A change in just one pHpoint represents a tenfold change in acidity. Normal rain has a pH of 5.6 due to acidic carbon dioxide in the air, but it is not harmful. Some rain has an acidity rating of 3 which is equivalent to the level of vinegar, a strongly acidic substance.

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The amount of acidity an area can withstand is dependent on the soil of the area. Some areas are much more susceptible to acid rain than others. In eastern Canada the soil contains very little lime - a natural base that neutralizes the acid in the rain. In the western areas of Canada the rocky soil is rich in natural bases, so it is less vulnerable to the environmental degradation of acid deposition. Scientists call the point where the environment is no longer able to absorb the pollution the critical load. Since different regions have different levels of pollution, they have varying critical load levels. Western Canada has a higher critical load than eastern Canada because of its ability to neutralize more pollution.

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Efforts to reduce pollution below the critical load values are being coupled with increasing the environment's capacity to tolerate acid rain. In New England, scientists are using calcium pellets to make the soil more basic. In areas where the population of maple trees has been on decline for more than 20 years, these pellets are offering a new hope. Areas that were injected with the pellets supported five times as many saplings as those that were not, proving that science can combat acid rain.

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Although there is still progress to be made, a lot has been accomplished already, and the problem is starting to disappear. It is important to remember that the current success in fighting acid rain is due to multi-lateral efforts. Reducing pollution and controlling emissions, while implementing new technologies, like the calcium pellets, are important ways to speed up recovery. Furthermore international cooperation has been essential. Joint efforts between the U.S. and Canada are already helping to resolve the acid rain dilemma. Approaching the problem through many angles with the help of science takes us one step closer to a world where it can never "acid rain" on our parades.

General Questions

Approxmiately what percentage of the acidity in the atmosphere falls back to the earth in the form of dry deposition?

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A. roughtly 100%B. Roughly 50%C. Roughtly 25%

In 1998, the United States was the primary contributor to the amount of SO2 emissions in North America. These emissions are principally generated by

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A. US industrial sourcesB. US electrical utilitiesC. US Fuel combustionD. US Transportation

In
Figure 9. Canada/U.S. Total NOx Emisions, 1990-2010
which was released in 2005, the section of the graph from 2005 to 2010 was

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A. extrapolated from the data trends obtainined between 1998 and 2005.B. obtained from secret government reportsC. interpolated from the data between 1998 and 2005.D. an graphic designer's optimistic conjecture

In paragraph 9 it is stated that normal rain has a pH of 5.6 caused by acidic carbon dioxide normally found in the air. How much more acidic is acid rain reported at a pH of 5?

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A. approximately 1/4th as many [H+] B. approximately 0.8 times as many [H+] C. approximately 1.25 times as many [H+] D. approximately 40 times as many [H+]

In 1998, Canada emitted 2.7 metric tons, or tonnes, of NOx. This is equivalent to how many American tons?

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A. exactly the same number, 2.7 American tonsB. Roughly 2.45 American tonsC. Roughly 2.97 American tons

Which of the following statements INCORRECTLY describes why some areas of Canada and the United States less susceptical to being damaged by acid rain?

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A. The addition of calcium pellets reduces the affect of acid rain by making the soil more alkaline.B. Areas in which there is less lime in the soil (which is a base) are not as easily damaged by acid rainC. The presence of rocky soil that contains more naturally occuring bases.D. Different levels of local pollution affect the amount of acid rain that can be accommodated without damaging the area.